4071-AutoZero, ADC Calibration time

[Toni2]

Hi!

For DMM 4071 i need following information:
- Length of AutoZero period if is enabled
- Length of ACD calibration period if is enabled

Thanks!

[Claudia]

Hi Tony2,

The period of Autozero and ADC Calibration depends on the configuration of your measurement. So the best way for you to know how long they take is to benchmark it.

Throughout this message I will make references to the NI Digital Multimeters Help file, accessible via the Start menu in your PC or at the following link:
NI Digital Multimeters Help

Attached to this message I am sending you an example program you can use to benchmark your measurement period based on your configuration.
This example program uses simluation of a PXI-4071, so you can run it even if you do not have a PXI-4071 installed in your computer. You could also simulate other boards, like 4070, 4072, etc.

• For more information on simulating DMMs, refer to the section "Programming with NI-DMM >> Features >> Simulating NI Digital Multimeters" of the NI Digital Multimeters Help file.

To find out the measurement period, this example program uses the function niDMM Get Measurement Period. Notice that this function is used after all the configuration functions have already executed. If this function was placed before, your measurement period results would not be correct.

So let me give you an example. Imagine that you configure the PXI-4071 with the following settings:

• Function: DC Volts
• Range: 10V
• Resolution: 6 1/2 digits
• AutoZero: ON
• ADC Cal: ON
• Trigger Source: Immediate
• Trigger Count: 1
• Sample Trigger: Immediate
• Sample Trigger: 10

Then your measurement period would be 235.6ms.

The measurement cycle of the DMM consists of several parts. For a DC V measurement it consists of Switch time, AutoZero, ADC Cal, Settling Time and Aperture Time.

• For more information about the DMM Measurement Cycle refer to the section "Devices>>NI 4071>>DMM Measurements>>DMM Measurement Cycle" in the NI Digital Multimeters Help file.

Given that you configured the board for 6 1/2 digit resolution, DC V in the 10V range, then NI-DMM will configure for you the values for the Aperture Time and Settling Time. The default values used by the DMM driver are listed in the NI Digital Multimeters Help file.

• For more information about the DMM Measurement Defaults, refer to the section "Devices>>NI 4071>>DMM Measurements>>DMM Measurement Defaults" in the NI Digital Multimeters Help file.
  Note: You can configure the Aperture Time and Settling Time programmatically using property nodes.

You know now that in a DC 6 1/2 digit configuration the Aperture Time is set to 100ms, and for a DC V measurement in the 10V range the default Settling Time is 1ms.
So now you disable ADC Calibration and run your program again. Now your measurement period is 168.5ms. If you subtract your initial measuremnet period from this one, you get that AutoZero takes 67.1ms (235.6ms-168.5ms=67.1ms).
Therefore, ADC Calibration takes 235.6ms-100ms-1ms-67.1ms=67.5ms.

Keep in mind that the period of AutoZero and ADC Cal will vary depending on the configuration of your measurement. So if you change the configuration settings (function, range, resolution, etc), you will get different periods.

Note: The first point of a multipoint acquisition (equivalent to a single point acquisition) takes slightly longer than subsequent samples. In this example, 235.6ms was the period of the subsequent samples. To find out how long the first point takes, configure Trigger Count and Sample Count equal to 1.

Please feel free to reply to this message if you have any questions.

Claudia L
DMM R&D
National Instruments

[HerveG]

First of all happy new year 2006 to NI users and  developpers.
I have a  problem concerning  the  measurement time  of the NI 4071 7.5  digits DMM. I read the post by Claudia concerning the Autozero time and I also used the Vi to measure the "aperture time" of the ADC within the 4071. I measured around 1000 ms for 7.5 digits and smaller times for lower resolution measurement.
Nevertheless the specifications of the 4071 say that it is able to get 7.5 digits or 23 bits of resolution  for reading rates up to 7 sample/s if I remember well and according to figure 5 in the "understanding the NI PXI-4071 7.5-digit FlexxDMM architecture" at least superior to 1 sample/s which is what I measure.
Do I mis-understand when i read "Reading rate" for a DMM I understand : "how many different measure can I get per second" ?
Do I misuse the vi you provide or are they involving configuration and self tests that slow down the "reading rate" ??
Let me know
Sincerly
Hervé G (Grenoble, France)

[Mark_S1]

Thank you and Happy New Year to you to.

There is an example that ships with NI-DMM that shows how to acheive 7 reading/sec.  In LabVIEW it's called Max DC Reading rate at 7.5 digits.vi and is available for other languages as well.  The reasons the time you are getting with the driver is different are 1) Because we could not make any assumptions about the users signal and 2) We give the user the best accuracy and resolution by default.  The example shows how to still achieve 7 1/2 digits at a faster reading rate by making some assumptions about the signal.  We lower the total aperture and turn off ADC Cal.  You can minimize the impact of turning off ADC Cal by performing a SelfCal prior to aking the measurements.


Good luck,
Mark
NI-DMM Software